GossipRouter: support GET and REGISTER via UDP datagram packets

Currently, GossipRouter supports only TCP. If a GossipClient calls register() or getMembers(), then a TCP connection is established and torn down after the request. This is inefficient and leads to many sockets in TIME_WAIT states (which are cleaned up after 2 * MSL seconds).

If we (in GossipRouter) add an additional listener, which listens on a UDP port, and provide the same for GossipClient, we would not use precious TCP sockets.

Think about: we should separate the transport in GR from the processing logic, so we could support TCP, UDP, HTTP etc. Should we use JGroups (in unicast mode) for this ?